The present invention relates to a home position detecting apparatus for use in a printing apparatus having a print wheel.
Hitherto, there are known home position detecting apparatuses in which, for example, as shown in FIG. 1, a light emitting element 1 and a photoelectric element 2 are arranged to face each other through a print wheel PW at a position where a detecting hole DH formed at the home position of the print wheel passes so that the home position of this print wheel can be detected. In this case, the use of the structure of light emitting element 1 and photoelectric element 2 which are integrally formed can present a high detection accuracy since the perfect positional relationship is obtained between those elements. However, it is difficult to form such an integral structure of light emitting element 1 and photoelectric element 2 due to the mechanical limitation with the print wheel interposed therebetween. Therefore, light emitting element 1 and photoelectric element 2 are separately assembled and the relative positional relationship therebetween is adjusted.
However, such a separate assembly of light emitting element 1 and photoelectric element 2 has the following problem due to variations in sensitivities of those elements and due to the positional relationship therebetween. Namely, the output characteristic of the detecting apparatus may vary within a range determined by broken lines A and B in FIG. 2. Thus, if a detection level DL is set in accordance with an output characteristic B and then the detecting apparatus having an output characteristic A is used, there is a possibility that not only a normal home position HP but also adjacent step positions (HP+l) and (HP-1) are detected. Consequently, the step position adjacent to the normal home position is regarded as the home position. In FIG. 2, a solid line C represents the output characteristic of the detecting apparatus in the case where the sensitivities of light emitting element 1 and photoelectric element 2 are normal and also the positional relationship therebetween is normal.